An organic light emitting device is composed of two opposing electrodes and multilayered organic material thin films having semiconductor-like properties which are interposed therebetween. The organic light emitting device having the configuration uses a phenomenon in which electric energy is converted into light energy by using an organic material, that is, an organic light emitting phenomenon. Specifically, when voltage is applied between two electrodes in a structure in which an organic material layer is disposed between an anode and a cathode, holes from the anode and electrons from the cathode are injected into the organic material layer. When the injected holes and electrons meet each other, an exciton is formed, and the exciton falls down to a bottom state to emit light.
In the aforementioned organic light emitting device, light generated from the organic material layer is emitted through a light transmissive electrode, and the organic light emitting device may be typically classified into a top emission type, a bottom emission type, and a dual emission type. In the case of the top emission or bottom emission type, one of two electrodes needs to be a light transmissive electrode, and in the case of the dual emission type, both the two electrodes need to be a light transmissive electrode.
In respect to the aforementioned organic light emitting device, many studies have been concentrated since Kodak Co., Ltd., announced that when a multilayer structure is used, the device may be driven at a low voltage, and recently, a natural color display using the organic light emitting device is attached to a mobile phone and commercialized.
Further, as recent studies on the organic light emitting device using a phosphorescent material instead of an existing fluorescent material have been conducted, efficiency has rapidly improved, and it is also expected that the device would be able to replace an existing illumination in the near future.
In order to use the organic light emitting device as illumination, the device needs to be driven with high brightness unlike the existing natural color display, and to maintain constant brightness like the existing illumination. In order to sufficiently improve the brightness of the organic light emitting device, light emission needs to be implemented in a large area, and in order to implement light emission in the large area, a high driving current needs to be used. In addition, in order to maintain constant brightness in the large area, the aforementioned high current needs to be uniformly injected into the device having the large area.
In general, as an anode material of the organic light emitting device, a metal oxide having a large work function is usually used. However, electrical conductivity of the metal oxide is not relatively high. Therefore, when such a metal oxide is used in an organic EL or an LCD having a small display area, there is no problem, but when the metal oxide is used in a large area organic EL for use in illumination equipment, a voltage drop due to high current is so high that the current is not uniformly injected into a light emitting surface, and thus light emission of the device does not uniformly occur. For example, light is emitted only in the vicinity of a portion in which an electrode is electrically connected to a driving circuit, and weak light emission may occur or light emission may not occur in the other regions.
Meanwhile, as a cathode material of the organic light emitting device, a metal having a small work function or an alloy thereof is usually used. The aforementioned metal may have high electrical conductivity of the material itself, but when transparency of the electrode is required as a characteristic of the organic material device, electrical conductivity is reduced if the metal is formed as a thin film. Therefore, even in this case, since current is not uniformly injected into a light emitting surface, light emission of the device may not uniformly occur.
Therefore, in order to use an organic light emitting device as illumination equipment, it is necessary to allow light emission with high brightness to uniformly occur in a device having a large area by reducing a resistance of an electrode.